1. Field of the Invention
The present invention relates to a device for minimizing the current consumption of a motor vehicle generator from the battery during the starting phase of the motor vehicle
2. Description of Related Art
It is known that one may control the output voltage of a generator using a generator regulator. The output voltage of a generator is a function of several influencing variables, to which belong the rotational speed, the electrical load in the vehicle electrical system, the charging state of the starter battery and the temperature. In order to generate a constant voltage in the vehicle electrical system, the generator regulator regulates the output voltage within specified limits. This regulation takes place via an adjustment of the excitation current flowing through the excitation winding of the generator. In order to adjust the excitation current, a change in the pulse duty factor of a PWM control signal made available by the regulator control for a switching transistor is carried out.
In the use of a high-side transistor as switching transistor of the generator regulator, the excitation current is taken away from a DC supply connection B+, and is guided via the transistor to the excitation winding. The second connection of the excitation winding is connected to ground.
The alternating voltages generated at phase voltage connections U, V and W are rectified in a rectifier device having a plurality of branches, and are provide to the vehicle electrical system of the motor vehicle and also conducted to the above-mentioned direct-voltage supply connection.
Furthermore, the generator regulator is also connected to at least one of the phase voltage connections of the generator, in order to evaluate at least one of the phase voltages of the generator.
The generator regulator has a regulator control which supplies a switching signal for the switching transistor of the generator regulator, and to which, as the input signal, present at the direct voltage supply connection B+ as well as the at least one of the phase voltages of the generator are supplied.
It is known that, at an engine start using the generator regulator, one may generate a PWM control signal having a specified fixed pulse duty factor, in order to provide an excitation current. This is a condition in order, at the engine start, to be able to ensure a certain rotational speed detection. The pulse duty factor mentioned is selected so that an engine start is able to be certainly detected in all temperature ranges using a rotational speed detection via at least one phase voltage. The phase voltage, in this instance, has to have a sufficiently great amplitude.
The amplitude of the phase voltage is a function of the excitation current, and with that, in the case of a fixedly set pulse duty factor, also of the temperature, by which an ohmic resistance, provided in the excitation circuit, is affected.
In order to assure a certain rotational speed evaluation in the engine start phase at all temperatures, the pulse duty factor in the standstill phase has to be set corresponding to the unfavorable conditions.
However, this has the result that, in case of the presence of more suitable working points, such as are present at lower temperatures, at the fixedly set pulse duty factor a larger excitation current comes about, and with that, a greater current consumption from the battery of the vehicle than would be necessary.
From published German patent application document DE 69213538 T2, a monitoring circuit is known for a device for reloading a battery in a vehicle, which prevents a high current consumption when the motor vehicle is standing still or the engine is shut down. The monitoring circuit mentioned includes a sensor stage which supplies a signal that indicates a current flowing in the field coil. The current flow through the control transistor of the field coil is regulated so that there exists a first or a second operating mode. In the first operating mode, which is initiated when the rotational speed of the generator is less than a specified level, a peak current, having a low average value, flows in the field coil. In the second operating mode, which is initiated when the rotational speed of the generator is greater than the specified level, a peak current, having a higher average value, flows in the field coil. In the known monitoring circuit, as a result, the current flowing through the field coil is modified as a function of the speed of the rotor of the generator.